APPLICATIONS INFORMATION
Introduction
AtypicalcircuitapplicationisshowninFigure1,above.
This is suitable for 75⍀ transmission line connections
at both the input and the output and is useful for
distribution of wide-band signals such as video via
cables. The 75⍀ reverse terminating resistor R4 gives
the correct matching condition to a terminated video
cable. The amplifier load is then 150⍀ in parallel with
the local feedback network.
The wide bandwidth of this device necessitates some
care in the layout of the printed circuit. A continuous
ground plane is required under the device and its
signal connection paths, to provide the shortest
possible ground return paths for signals and power
supply filtering. A double-sided or multi-layer PCB
construction is required, with plated-through via holes
providing closely spaced low-inductance connections
from some components to the continuous ground
plane.
For the power supply filtering, low inductance surface
mount capacitors are normally required. It has been
found that very good RF decoupling is provided on
each supply using a 1000pF NPO size 0805 or smaller
ceramic surface mount capacitor, closest to the device
pin, with an adjacent 0.1uF X7R capacitor. Other
configurations are possible and it may be found that a
single 0.01uF X7R capacitoron eachsupply gives good
results. However this should be supported by larger
decoupling capacitors elsewhere on the printed circuit
board. Values of 1 to 10µF are recommended,
particularly where the voltage regulators are located
more than a few inches from the device. These larger
capacitors are recommended to be solid tantalum
electrolytic or ceramic types.
Note particularly that the inverting input of this current
feedback type of amplifier is sensitive to small
amounts of capacitance to ground which occur as part
of the practical circuit board layout. This capacitance
affects bandwidth, frequency response peaking and
pulse overshoot. Therefore to minimise this
capacitance, the feedback components R2 and R3 of
Figure1 should be positioned as close as possible to
the inverting input connection.
The frequency response and pulse response will vary
according to particular values of resistors and layout
capacitance. The response can be tailored for the
application to some extent by choice of the value of
feedback resistor. Figure 2 shows an oscilloscope
display of the pulse response for a practical double
sided printed circuit board where RF = 510⍀.
ZXFV201, ZXFV203
SEMICONDUCTORS
ISSUE 3 - DECEMBER 2003
4
Figure 3: Graphs of gain and phase vs frequncy (R
L
=150 )
